Network Working Group J. Polk
Request for Comments: 5432 S. Dhesikan
Category: Standards Track Cisco Systems
G. Camarillo
Ericsson
March 2009
Quality of Service (QoS) Mechanism Selectionin the Session Description Protocol (SDP)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
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Polk, et al. Standards Track [Page 1]

RFC 5432 QoS Mechanism Selection in SDP March 20091. Introduction
The offer/answer model [RFC3264] for SDP [RFC4566] does not provide
any mechanism for endpoints to negotiate the QoS mechanism to be used
for a particular media stream. Even when QoS preconditions [RFC3312]
are used, the choice of the QoS mechanism is left unspecified and is
up to the endpoints.
Endpoints that support more than one QoS mechanism need a way to
negotiate which one to use for a particular media stream. Examples
of QoS mechanisms are RSVP (Resource Reservation Protocol) [RFC2205]
and NSIS (Next Steps in Signaling) [QoS-NSLP].
This document defines a mechanism that allows endpoints to negotiate
the QoS mechanism to be used for a particular media stream. However,
the fact that endpoints agree on a particular QoS mechanism does not
imply that that particular mechanism is supported by the network.
Discovering which QoS mechanisms are supported at the network layer
is out of the scope of this document. In any case, the information
the endpoints exchange to negotiate QoS mechanisms, as defined in
this document, can be useful for a network operator to resolve a
subset of the QoS interoperability problem -- namely, to ensure that
a mechanism commonly acceptable to the endpoints is chosen and make
it possible to debug potential misconfiguration situations.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. SDP Attributes Definition
This document defines the 'qos-mech-send' and 'qos-mech-recv' session
and media-level SDP [RFC4566] attributes. The following is their
Augmented Backus-Naur Form (ABNF) [RFC5234] syntax, which is based on
the SDP [RFC4566] grammar:
attribute =/ qos-mech-send-attr
attribute =/ qos-mech-recv-attr
qos-mech-send-attr = "qos-mech-send" ":"
[[SP] qos-mech *(SP qos-mech)]
qos-mech-recv-attr = "qos-mech-recv" ":"
[[SP] qos-mech *(SP qos-mech)]
qos-mech = "rsvp" / "nsis" / extension-mech
extension-mech = token
Polk, et al. Standards Track [Page 3]

RFC 5432 QoS Mechanism Selection in SDP March 2009
The 'qos-mech' token identifies a QoS mechanism that is supported by
the entity generating the session description. A token that appears
in a 'qos-mech-send' attribute identifies a QoS mechanism that can be
used to reserve resources for traffic sent by the entity generating
the session description. A token that appears in a 'qos-mech-recv'
attribute identifies a QoS mechanism that can be used to reserve
resources for traffic received by the entity generating the session
description.
The 'qos-mech-send' and 'qos-mech-recv' attributes are not
interdependent; one can be used without the other.
The following is an example of an 'm' line with 'qos-mech-send' and
'qos-mech-recv' attributes:
m=audio 50000 RTP/AVP 0
a=qos-mech-send: rsvp nsis
a=qos-mech-recv: rsvp nsis
4. Offer/Answer Behavior
Through the use of the 'qos-mech-send' and 'qos-mech-recv'
attributes, an offer/answer exchange allows endpoints to come up with
a list of common QoS mechanisms sorted by preference. However, note
that endpoints negotiate in which direction QoS is needed using other
mechanisms, such as preconditions [RFC3312]. Endpoints may also use
other mechanisms to negotiate, if needed, the parameters to use with
a given QoS mechanism (e.g., bandwidth to be reserved).
4.1. Offerer Behavior
Offerers include a 'qos-mech-send' attribute with the tokens
corresponding to the QoS mechanisms (in order of preference) that are
supported in the send direction. Similarly, offerers include a
'qos-mech-recv' attribute with the tokens corresponding to the QoS
mechanisms (in order of preference) that are supported in the receive
direction.
4.2. Answerer Behavior
On receiving an offer with a set of tokens in a 'qos-mech-send'
attribute, the answerer takes those tokens corresponding to QoS
mechanisms that it supports in the receive direction and includes
them, in order of preference, in a 'qos-mech-recv' attribute in the
answer. On receiving an offer with a set of tokens in a 'qos-mech-
recv' attribute, the answerer takes those tokens corresponding to QoS
mechanisms that it supports in the send direction and includes them,
in order of preference, in a 'qos-mech-send' attribute in the answer.
Polk, et al. Standards Track [Page 4]

RFC 5432 QoS Mechanism Selection in SDP March 2009
When ordering the tokens in a 'qos-mech-send' or a 'qos-mech-recv'
attribute by preference, the answerer may take into account its own
preferences and those expressed in the offer. However, the exact
algorithm to be used to order such token lists is outside the scope
of this specification.
Note that if the answerer does not have any QoS mechanism in common
with the offerer, it will return empty 'qos-mech-send' and 'qos-mech-
recv' attributes.
4.3. Resource Reservation
Once the offer/answer exchange completes, both offerer and answerer
use the token lists in the 'qos-mech-send' and 'qos-mech-recv'
attributes of the answer to perform resource reservations. Offerers
and answerers SHOULD attempt to use the QoS mechanism with highest
priority in each direction first. If an endpoint (the offerer or the
answerer) does not succeed in using the mechanism with highest
priority in a given direction, it SHOULD attempt to use the next QoS
mechanism in order of priority in that direction, and so on.
If an endpoint unsuccessfully tries all the common QoS mechanisms for
a given direction, the endpoint MAY attempt to use additional QoS
mechanisms not supported by the remote endpoint. This is because
there may be network entities out of the endpoint's control (e.g., an
RSVP proxy) that make those mechanisms work.
4.4. Subsequent Offer/Answer Exchanges
If, during an established session for which the QoS mechanism to be
used for a given direction was agreed upon using the mechanism
defined in this specification, an endpoint receives a subsequent
offer that does not contain the QoS selection attribute corresponding
to that direction (i.e., the 'qos-mech-send' or 'qos-mech-recv'
attribute is missing), the endpoints SHOULD continue using the same
QoS mechanism used up to that moment.
5. Example
The following is an offer/answer exchange between two endpoints using
the 'qos-mech-send' and 'qos-mech-recv' attributes. Parts of the
session descriptions are omitted for clarity purposes.
The offerer generates the following session description, listing both
RSVP and NSIS for both directions. The offerer would prefer to use
RSVP and, thus, includes it before NSIS.
Polk, et al. Standards Track [Page 5]

RFC 5432 QoS Mechanism Selection in SDP March 2009
Subject to charset: No
Purpose of attribute: To list QoS mechanisms supported in the
receive direction
Allowed attribute values: IANA Registered Tokens
6.3. Registry for QoS Mechanism Tokens
The IANA has created a subregistry for QoS mechanism token values to
be used in the 'qos-mech-send' and 'qos-mech-recv' attributes under
the Session Description Protocol (SDP) Parameters registry. The
initial values for the subregistry are as follows:
QoS Mechanism Reference
---------------------------- ---------
rsvp RFC 5432
nsis RFC 5432
As per the terminology in [RFC5226], the registration policy for new
QoS mechanism token values shall be 'Specification Required'.
7. Security Considerations
An attacker may attempt to add, modify, or remove 'qos-mech-send' and
'qos-mech-recv' attributes from a session description. This could
result in an application behaving in a non-desirable way. For
example, the endpoints under attack may not be able to find a common
QoS mechanism to use.
Consequently, it is strongly RECOMMENDED that integrity and
authenticity protection be applied to SDP session descriptions
carrying these attributes. For session descriptions carried in SIP
[RFC3261], S/MIME [RFC3851] is the natural choice to provide such
end-to-end integrity protection, as described in [RFC3261]. Other
applications MAY use a different form of integrity protection.
8. Acknowledgements
Dave Oran helped form this effort. Flemming Andreasen and Magnus
Westerlund provided useful comments on this specification.
Polk, et al. Standards Track [Page 7]